Year 11 Module B4 Revision notes. Plants, photosythesis, biomass and the carbon cycle.

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Year 11 Module B4 Revision notes

Photosynthesis:

                                              Light

Carbon dioxide + water                                          glucose + oxygen

                                                 Chlorophyll

Leaves

  • Designed for making food by photosynthesis.

Leaves are adapted for efficient photosynthesis:

  • They are broad – large surface area exposed to light
  • Thin- carbon dioxide and water vapour only have a short distance to reach palisade cells for photosynthesis.
  • There are air spaces in the spongy Mesophyll layer – allows carbon dioxide and oxygen to diffuse easily between cells, large surface area for gas exchange.  There is a big internal surface area to volume ratio.
  • Leaves contain lots of chlorophyll, found in lots of chloroplasts, found in the palisade layer
  • Upper epidermis is transparent – light can pass through it into the palisade layer.
  • Lower surface of leaf is full of little holes called stomata; let carbon dioxide and oxygen in and out; also let water escape (transpiration).
  • Leaves have a network of veins – deliver water and nutrients to every part of the leaf and take food produced by the leaf away; they also help to support leaf structure.

Palisade cells

Packed with chloroplasts; tall shape means a lot of surface area is exposed down the side for absorbing carbon dioxide from the air in the leaf; shape also means good chance of light hitting a chloroplast.

Plants exchange gases by diffusion.

  • Diffusion is the passive movement of particles from an area of higher concentration to an area of lower concentration.

Osmosis

  • A special kind of diffusion.

Osmosis is the movement of water molecules across a partially permeable membrane from a region of higher water concentration to a region of lower water concentration.

A partially permeable membrane is one with very small holes in it – water can pass through but larger molecules can’t.

The water molecules can pass through both ways – they move randomly all the time.  Because there are more water molecules on one side than the other, there is a steady net flow of water into the region with fewer water molecules, i.e. water moves into the stronger sucrose solution.  This means the concentrated sucrose solution gets more dilute.

Turgor pressure – supports plant tissues

When a plant is well watered, all its cells will draw water in by osmosis, they become plump and swollen.  When cells are like this they are said to be turgid.  The contents of the cell push against the cell wall – this is called turgor pressure.  Turgor pressure helps to support the plant tissues.

If there is no water in the soil a plant starts to wilt.  The cells start to lose water and they lose their turgor pressure.  The cells are flaccid.

If a plant is really short of water the cytoplasm inside the cell starts to shrink and the membrane pulls away from the cell wall.  The cell is now plasmolysed; however the plant doesn’t totally lose shape because the inelastic cell wall keeps things in position.

Animal cells do not have a cell wall.  If an animal cell takes in too much water it bursts - this is known as lysis.  If an animal cell loses too much water it shrivels up – this is known as crenation.

Water flow through plants

Root hairs take in water by osmosis

Root hairs give the plant a large surface area for absorbing water from the soil.

There is usually a higher concentration of water in the soil than the plant so water is drawn into the root hair cell by osmosis.

Transpiration is the loss of water from the plant.

Transpiration is caused by the evaporation and diffusion of water from inside the leaves.

This creates a slight shortage of water in the leaf; more water is drawn up from the rest of the plant through the xylem vessels.  

More water is then drawn up from the roots.  There is a constant transpiration stream of water through the plant.

Transpiration is a “side-effect” of the way leaves are adapted for photosynthesis.  The stomata are essential for gas exchange, but this means that because there is more water inside the plant than in the surrounding air, water escapes from the leaves through the stomata.

Benefits of transpiration

  • The constant stream of water from the plant helps keep the plant cool
  • The plant is provided with a constant supply of water for photosynthesis
  • The water creates turgor pressure in the plant cells – this helps support the plant and prevents wilting.
  • Minerals needed by the plant can be brought in from the soil along with the water.

What affects transpiration?

  • Light intensity – the brighter the light the greater the transpiration rate.  Stomata begin to close as it gets darker.  Photosynthesis can’t happen in the dark so they don’t need to let carbon dioxide in. This means water can’t escape.
  • Temperature – the warmer it is the greater the transpiration rate.  This is because the water particles have more energy to evaporate and diffuse out of the stomata.
  • Air movement – lots of air movement (wind) around a leaf means faster transpiration.  If the air is still the water vapour surrounding the leaf doesn’t move away, this means there is a higher concentration of water outside of the leaf as well as inside it, hence diffusion doesn’t happen as quickly.  If it’s windy the water vapour is quickly moved away, this means there is a greater concentration difference and diffusion will happen more quickly.
  • Air humidity – if the air around the leaf is very dry transpiration happens more quickly.  If the air is humid there is a lot of water vapour already in it so there is less of a concentration difference between the surrounding air and that in the leaf – hence a slower rate of diffusion.
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Achieving the balance

Plants have adaptations to help reduce water loss

  • Leaves have a waxy cuticle covering their upper epidermis; this means the upper surface of the leaf is waterproof.
  • Most stomata are on the underside of the leaf where it is darker and cooler.  This helps slow down the diffusion of water out of the leaf.
  • Bigger stomata and lots of stomata mean more water loss.  In hot climates plants have fewer, smaller stomata on the underside of the leaf and no stomata on the upper epidermis.

Stomata open and close automatically

When supplies of ...

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**** A very detailed set of revision notes. A good grasp of concepts demonstrated.